Ion image to electron image converter



Oct- 4, 1966 A. T. FORRESTER ETAL 3,277,297

ION IMAGE TO ELECTRON IMAGE CONVERTER Filed April 29, 1963 1y, 1

ALVIN 7. FORRESTER ARTHUR B. MARcHAA r A TTORNEY 3 the desired visible image.

United States Patent r 3,277,297 ION IMAGE TO ELECTRON IMAGE CONVERTER Alvin T. Forrester, Los Angeles, and Arthur B. Marchant, Pasadena, Calif., assignors to Electra-Optical Systems, Inc., Pasadena, Calif.

Filed Apr. 29, 1963, Ser. No. 276,625 3 Claims. (Cl. 250--49.5)

The present invention relates to a new and novel apparatus for producing an ion image and of thereafter converting it to a corresponding electron image.

There are a number of instances when it is or would be desirable to obtain an ion rather than an electron image. For example, a great deal of research and development effort has been expended in recent years in attempting to produce a feasible ion engine. Toward this end, it would certainly be helpful if the pattern of the ion beam emitted could be readily obtained so that it could be studied, with the objective in mind of improving the beam. Again, in the field of microscopy, high-field ion emission gives much better resolution than high-field electron emission, principally because high mass particles have much smaller wavelengths than electrons of the same energy.

However, ions have a deteriorating effect on the surface of fluorescent screens on which they are made to impinge. Furthermore, because of their mass and relatively low velocity, ions produce very low light levels, that is to say, they produce rather di'm images. Stated otherwise, ions dissipate their energy in heat and damage to the fluorescent screen surface whereas electrons are much more effective in producing light. Consequently, whatever the purpose, it is preferred that the ion-beam pattern be converted to a corresponding electron-beam pattern and to .then produce the visible pattern or image using vention provides a way for achieving this result.

More particularly, according to an embodiment of the invention, an ion source is made to produce an ion beam whose pattern at any time reflects the nature of the ion source surface at that time. The ion beam thusly produced is then focused upon a specially constructed grid structure. Upon striking the grid, the ions dislodge-a larger number of electrons in the same pattern, a phenomenon known as secondary emission, and it is the electron-beam rather than the ion-beam image that is brought to bear upon the fluorescent screen to produce As a result, the screen surface is safeguarded and a' higher degree of brightness is obtained for the visible image than might ordinarily be obtained.

It is, therefore, an' object of the present invention to provide an iorimicroscope of high resolving power in which an ion image is formed into an electron image by the process of secondary emission.

It is a further object of the present invention to provide an apparatus by means of which the pattern of ion beams emitted from a surface may be visually examined.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose 'of illustration and description only and is not intended as a definition of the limits of the invention.

FIG. 1 is a diagrammatic sectional view of an embodiment of the invention and shows the electron image being formed from the ion image by the process of secondary emission; 1

FIG. 2 is a diagrammatic sectional view of an ion source that may be used in the FIG. 1 embodiment for studying ion emission from a surface;

FIG. 3 is a frontal view of a portion of the grid structure used in the FIG. 1 apparatus; and

FIG. 4 is a modified version of the FIG. 1 embodiment and illustrates a different kind of ion source than that shown in FIG. 2 for studying a surface from which ions are not normally emitted.

Referring now to the drawing and to the embodiment of FIG. 1 in particular, an evacuated vessel 10 is provided having a faceplate 10a made of glass or some other transparent material. On the inside surface of faceplate 10a is a luminous or fluorescent viewing screen 11 preferably made up of two layers designated 11a and 11b, layer 11a being made of a phosphor material that emits light when struck by electrons and layer 11b being a very thin metal film, such as aluminum, that covers the surface of the phosphor layer. The best results for image definition are obtained with grainless evaporated phosphors or with single large crystals of phosphors because of their grainless structure. Film llb is thin enough so that electrons can penetrate it and is made of metal so as to provide an equipotential surface over the phosphor layer. Aluminum is preferred as a film material in order to reflect light toward the glass faceplate.

At the other end of vessel 10, that is, at the end opposite viewing screen 11, is an ion source 12 from the front part of which are emitted ion-beam patterns. Suitably disposed in front of ion source 12 is a pair of focusing electrodes 13a and 13b which, as their name implies, focuses the ion beam onto a grid structure 14 positioned in front of and spaced from fluorescent screen 11. Since the construction, action and use of focusing electrodes are extremely well-known in the electronics art, it is not deemed necessary to decribe them in any detail here.

One ion source that may be employed in the FIG. 1 embodiment is shown in FIG. 2 and, as shown therein, it includes a chamber 15 having an inlet tube 15a that connects with the chamber through vessel 10. At the front end of the chamber is a relatively thick layer 16 of porous tungsten which forms the front wall of the chamber. To provide the required ion beam, hot cesium vapor 17 is forced to flow through inlet 15a and into chamber 15. Thereafter, the cesium vapor flows through porous tungsten 16 and, in so doing, becomes partially ionized, the pattern of the ion beam emerging from the tungsten being determined by the nature of the porous tungsten layer.

Grid 14 changes the ion stream to electrons by secondaryemission and a preferred arrangement of such a grid is shown in detail in FIG. 3. Although any one of several different metals may be used, the grid is preferably made of nickel and is solid except for a large number of tiny holes or openings, designated 18, arranged in columns and rows. In the example being presented, the holes through the grid are square-shaped 0.0005 inch to the side. In addition, the spacing between holes is uniformly 0.0005 inch. Thus, the grid shown in FIG. 3 is a 1,000 mesh nickel screen that is twenty-five percent open. One way of making such a grid is by means of the elect-roforming process, which is also well-known.

Finally, in order to establish a proper electric field between grid 14 and viewing screen 11, a voltage source 19 is connected between the grid and metal film 11b, source 19 being connected so as to keep grid 14 at a negative potential relative to that on film 1-1b. As an example of the magnitudes, of the potentials involved, grid 14 may be at a potential of 10,000 volts whereas the potential for film 11b may be -1000 volts or it may be at ground potential. It will be recognized from the potentials mentioned that electrons will be strongly attracted to viewing screen 11 whereas ions will be repelled by it.

In operation, a beam of ions, designated 20, is emitted in a pattern at ion source 12. By means of electrodes 13a and 13b, the beam is focused upon and projected against grid structure 14. Upon striking the grid surface, the ions dislodge electrons, the pattern of the electrons thusly produced being substantially identical to the pattern of the ions. This is the process of secondary emission previously mentioned. More specifically, if the ions strike the nearer surface of grid 14, then the electrons thereby dislodged from its surface reverse their direction and pass through holes 18, as is indicated by arrows 21a in FIG. 1. On the other hand, if the ions initially pass through the grid holes, they utirnately reverse their direction because of the field that exists between the grid and the viewing screen and fall back into the farther or opposite grid surface. In so doing, they also produce secondary emission of electrons, as is indicated by arrows 21b. As mentioned above, the pattern of these secondarily emitted electrons corresponds to the pattern of the ions.

Since the potential of metal film 11b is positive relative to the potential of grid 14, the beam of electrons is attracted toward viewing screen 11. As a result, the electrons strike and pass through metal film 11b and enter phosphor layer 11a wherein the energy of the electrons is in some part converted to light energy, the visible image or pattern thusly produced conforming or corresponding to the previous electron and ion patterns. Aluminum layer 11b aids in producing a sharp image since light incident upon it is reflected toward faceplate a.

In the embodiment described, the ion source is of the type illustrated in FIG. 2 in which the ions are produced by the source itself. However, for microscopy purposes, it would be necessary to modify the FIG. 1 apparatus to provide a different ion beam source, that is, of the kind shown in FIG. 4 wherein a beamof ions 22 is directed against'a specimen 23 whose surface characteristics are to be studied. Upon striking the specimen, the beam is reflected, the pattern of the reflected beam being determined by and, therefore, providing an indication of the specimen surface. The reflected beam is thereafter treated as was beam 20 before. Hence, except for the ion source, the remaining apparatus is the same. An ion source that can be adapted for use therein is shown and described in the patent to Gustav Weissenberg entitled Method and Apparatus for Obtaining Ultramicroscopic Images in an Ion Microscope, Patent No. 2,799,779, issued July 16, 1957.

Although a particular arrangement of the invention has been illustrated and described above by way of example, it is not intended that the invention be limited thereto. Accordingly, the invention should be considered to include any and all modifications, alterations or equivalent arrangements falling within the scope of the annexed claims.

Having thus described the invention, what is claimed is:

1. Ion-electron converter apparatus comprising: source means for producing an ion image of a surface; a fine metal grid structure for emitting an image of electrons in response to the impingement of a corresponding image of ions thereon; first means for projecting said ion image against said metal grid structure, whereby said electron image is emitted therefrom that corresponds to said ion image; a fluorescent viewing screen parallel to said grid structure; and second means for projecting said electron image onto said screen, whereby a visible image is produced that corresponds to said ion image.

2. Ion-electron converter apparatus comprising: an ion source including a chamber having one wall made of a porous metal that can withstand high temperatures, and hot cesium vapor flowing into said chamber and through said porous metal wall, whereby an ion image of said porous metal structure is formed; a fine metal grid structure for emitting an electron image in response to the impingement of an ion image thereon; first means for projecting said ion image against said metal grid structure, whereby said electron image is emitted therefrom that corresponds to said ion image; a fluorescent viewing screen parallel to said grid structure; and second means for projecting said electron image onto said screen, whereby a visible image is produced that corresponds to said ion image.

3. Ion-electron converter apparatus comprising: an evacuated envelope containing means for focusing an ion beam upon an object to be examined, the pattern of ions reflected from the object corresponding to the nature of its surface, a grid structure for emitting electrons in response to the impingement of ions thereon, said grid having a uniformly distributed array of holes therethrough through which said electrons may pass, means for focusing and projecting said .ion image onto said grid structure, whereby an electron image is emitted that corresponds to said ion image, a fluorescent viewing screen in face-to-face relationship with said grid structure, said screen including a phosphor layer that emits light in response to the impingement of electrons thereon, and a thin metal film through which electrons can pass, said metal film covering said phosphor layer on the side of said grid structure; and additional means for projecting said electron image against said screen, whereby a visible image is produced that corresponds to said ion image.

References Cited by the Examiner UNITED STATES PATENTS 2,570,124 10/1951 Hernquist 25041.9 2,799,779 7/ 1957 Weissenberg 25049.5 2,814,730 11/1957 Fechter 250495 2,944,146 7/1960 Schultz 313-63 X 2,992,358 7/1961 Farnsworth 250213 3,041,453 6/1962 Daly 2504l.9 3,096,456 7/1963 Shelton et a1. 31363 iRALPH G. NILSON, Primary Examiner.

E. STRICKLAND, J. D. WALL, Assistant Examin ers. 

1. ION-ELECTRON CONVERTER APPARATUS COMPRISING: SOURCE MEANS FOR PRODUCING AN ION IMAGE OF A SURFACE; A FINE METAL GRID STRUCTURE FOR EMITTING AN IMAGE OF ELECTRONS IN RESPONSE TO THE IMPINGEMENT OF A CORRESPONDING IMAGE OF IONS THEREON; FIRST MEANS FOR PROJECTING SAID ION IMAGE AGAINST SAID METAL GRID STRUCTURE, WHEREBY SAID ELECTRON IMAGE IS EMITTED THEREFROM THAT CORRESPONDS TO SAID ION IMAGE; A FLUORESECNE VIEWING SCREEN PARALLEL TO SAID GRID STRUCTURE; AND SECOND MEANS FOR PROJECTING SAID ELECTRON IMAGE ONTO SAID SCREEN, WHEREBY A VISIBLE IMAGE IS PRODUCED THAT CORRESPONDS TO SAID ION IMAGE. 